The present invention relates to a two-dimensional image detector capable of detecting images in radioactive rays (such as X-rays), visible light, infrared light, etc.
Conventionally, a known type of two-dimensional image detector for radioactive rays is a device comprising a two-dimensional arrangement of semiconductor sensors which detect X-rays and produce charge (electron-hole) pairs, each sensor being provided with an electrical switch, in which the electrical switches are sequentially turned ON by row, and the charge of each sensor in that row is read out.
The principle of such a two-dimensional image detector and specific structures therefor are discussed in, for example, D. L. Lee, et al, xe2x80x9cA New Digital Detector for Projection Radiographyxe2x80x9d (Physics of Medical Imaging, Proc. SPIE 2432, pp.237-249, 1995); L. S. Jeromin, et al, xe2x80x9cApplication of a-Si Active-Matrix Technology in a X-Ray Detector Panelxe2x80x9d (SID (Society for Information Display) International Symposium, Digest of Technical Papers, pp.91-94, 1997); and Japanese Unexamined Patent Publication No. 6-342098/1994 (Tokukaihei 6-342098, published on Dec. 13, 1994).
The following will explain the principle and structure of the foregoing conventional two-dimensional image detector for radioactive rays. FIG. 10 is a perspective view schematically showing the structure of the foregoing conventional two-dimensional image detector for radioactive rays. Further, FIG. 11 is a cross-sectional view schematically showing the structure of one pixel thereof.
As shown in FIGS. 10 and 11, the foregoing conventional two-dimensional image detector for radioactive rays includes an active matrix substrate made up of electrode lines (gate electrodes 52 and source electrodes 53) arranged in an XY matrix, TFTs (thin-film transistors) 54, charge storage capacitors (Cs) 55, etc., provided on a glass substrate 51. Further, over substantially the entire surface of the active matrix substrate 51 are provided a photoconductive film 56, a dielectric layer 57, and an upper electrode 58.
Each charge storage capacitor 55 is made up of a Cs electrode 59 and a pixel electrode 60 (which is connected to the drain electrode of the TFT 54), provided opposite each other but separated by an insulating layer 61.
The photoconductive film 56 is made of a semiconductor material which produces a charge (electron-hole) when radioactive rays such as X-rays are projected thereon; the examples discussed in the foregoing documents use amorphous selenium (a-Se), which has high dark resistance and good photoconductive characteristics for X-rays. The photoconductive film 56 is formed by vacuum vapor deposition with a thickness of 300 xcexcm to 600 xcexcm.
For the foregoing active matrix substrate, it is possible to use active matrix substrates formed in the process of manufacturing liquid crystal display devices. For example, an active matrix substrate used in an active matrix liquid crystal display device (AMLCD) has a structure which includes TFTs made of amorphous silicon (a-Si) or polycrystalline silicon (p-Si), electrodes arranged in an XY matrix, and charge storage capacitors (Cs). Accordingly, it is easy to use such an AMLCD as an active matrix substrate for a two-dimensional image detector for radioactive rays, necessitating only minor design changes.
The following will explain the operating principle of the foregoing conventional two-dimensional image detector for radioactive rays.
When radioactive rays are projected onto the photoconductive film 56, charge (electron-hole) pairs are produced therein. As shown in FIGS. 10 and 11, the photoconductive film 56 and the charge storage capacitor 55 are electrically connected in series, and thus if a voltage is applied across the upper electrode 58 and the Cs electrode 59 in advance, the electron and hole members of the charge pairs produced in the photoconductive film 56 move to the + and xe2x88x92 electrode sides, respectively. As a result, a charge (electron-hole) accumulates in the charge storage capacitor 55. Between the photoconductive film 56 and the charge storage capacitor 55 is provided an electron blocking layer 62 made of a thin insulating layer, which serves as a blocking photodiode for blocking a charge injection from one side to the other.
By putting the TFTs 54 in an open state by means of input signals from gate electrodes G1, G2, G3, . . . , Gn, the charges accumulated in the respective charge storage capacitors 55 due to the foregoing effect can be drawn out through source electrodes S1, S2, S3, . . . , Sn. Since the gate electrodes 52, the source electrodes 53, the TFTs 54, the charge storage capacitors 55, etc. are all provided in the form of an XY matrix, X-ray image information can be obtained two-dimensionally by sequential scanning of the signals inputted to the gate electrodes G1, G2, G3, . . . , Gn.
Incidentally, if the photoconductive film 56 used has photoconductivity not only for radioactive rays such as X-rays but also for visible light, infrared light, etc., the foregoing conventional two-dimensional image detector can also function as a two-dimensional image detector for visible light, infrared light, etc.
The foregoing conventional two-dimensional image detector for radioactive rays uses a-Se for the photoconductive film 56, but a-Se has the following drawbacks: due to the insufficient sensitivity to X-rays (S/N ratio) of a-Se, information cannot be read out unless the charge storage capacitors 55 are sufficiently charged by long X-ray exposure.
Further, in the foregoing conventional two-dimensional image detector for radioactive rays, a dielectric layer 57 is provided between the photoconductive film 56 and the upper electrode 58 in order to reduce current leakage (dark current) and to protect from high voltage. However, since it is necessary to add a step (sequence) for eliminating a residual charge from the dielectric layer 57 after each frame, another drawback of the foregoing conventional two-dimensional image detector for radioactive rays is that it can only be used for pickup of still images.
In order to obtain image data corresponding to moving images, on the other hand, it is necessary to use, instead of a-Se, a photoconductive film 56 which has superior sensitivity to X-rays (S/N ratio) . By improving the sensitivity of the photoconductive film 56, it is possible to sufficiently charge the charge storing capacitor 55 with X-ray exposure of short duration, and since a high voltage need not be applied to the photoconductive film 56, the dielectric layer 57 itself is no longer necessary.
Known examples of this kind of photoconductive material with superior sensitivity to X-rays include CdTe and CdZnTe. Since photoelectric absorption of X-rays by a substance is generally proportional to the fifth power of its effective atomic number, if, for example, the effective atomic number of Se is 34 and that of CdTe is 50, then CdTe can be expected to have a sensitivity of approximately 6.9 times that of Se. However, if replacement of the a-Se in the photoconductive film 56 of the foregoing conventional two-dimensional image detector with CdTe or CdZnTe is attempted, the following problems arise.
With conventional a-Se, a film can be formed by vacuum vapor deposition, and in this case, since the film formation temperature can be set at room temperature, it is easy to form a film on the foregoing active matrix substrate. With CdTe or CdZnTe, on the other hand, film formation by MBE (molecular beam epitaxy) or MOCVD (metal organic chemical vapor deposition) are known; in view of film formation on, in particular, substrates of large surface area, MOCVD is considered most suitable.
However, when forming a film of CdTe or CdZnTe by MOCVD, since the starting materials organic cadmium and organic tellurium have heat decomposition temperatures of approximately 300xc2x0 C. (for dimethyl cadmiumxe2x80x94DMCd) and approximately 400xc2x0 C. (for diethyl telluriumxe2x80x94DETe) or approximately 350xc2x0 C. (for diisopropyl telluriumxe2x80x94DiPTe), a high film formation temperature of around 400xc2x0 C. is needed.
The TFTs 54 formed on the, foregoing active matrix substrate generally use semiconductor layers made of films of a-Si or p-Si, which, in order to improve semiconductor characteristics, are formed while adding hydrogen (H2) at a film formation temperature of 300xc2x0 C. to 350xc2x0 C. The heat resistance of a TFT element formed in this manner is approximately 300xc2x0 C., and exposure to higher temperatures causes the hydrogen to escape from the a-Si or p-Si film, thus impairing semiconductor characteristics.
Accordingly, in consideration of film formation temperature, it is in fact difficult to use MOCVD to form a film of CdTe or CdZnTe on the foregoing active matrix substrate.
To solve these problems, the two-dimensional image detector may be manufactured by forming the active matrix substrate and the photoconductive material layer (semiconductive layer) on different substrates in advance and thereafter laminating the substrates together. Here, the following two methods of laminating the substrates together are considered feasible in order to prevent cross-talk between neighboring pixels. The first method is such that conductivity is achieved only across the two substrates laminated together, while an anisotropic conductive material having an insulating property is used so as to achieve insulation in the substrate plane direction. The second method is such that a conductive material is used independently at each pixel in laminating the substrates together.
These methods make it possible to use semiconductor materials such as CdTe and CdZnTe in the foregoing two-dimensional image detector; such semiconductor materials conventionally could not been used directly on the active matrix substrate in consideration of the film formation temperature of semiconductors and the thermal resistance of the active matrix substrate.
However, these methods lead to problems as described below.
The following description explains the problems of the first method.
Appropriately adapted as the anisotropic conductive material used in the first method is an insulating adhesive (binder resin) in which conductive particles are dispersed, that is, a so-called anisotropic conductive adhesive. Applicable as conductive particles used in the anisotropic conductive adhesive are metal particles such as Au (gold)-plated Ni (nickel) particles, carbon particles, metal-coated plastic particles such as Au/Ni-plated plastic particles, conductive particle composite plastic obtained by mixing in polyurethane transparent conductive particles such as ITO particles as well as Ni particles, etc. As the adhesive, a heat-hardening type, photo-hardening type, or thermoplastic type of adhesive can be used.
The anisotropic conduction principle of the commonly used anisotropic conductive adhesive is described below with reference to FIGS. 12(a) through 12(c).
First of all, as shown in FIG. 12(a), an anisotropic conductive adhesive 64 is applied over one of the substrates to be laminated together (an active matrix substrate 63 in this case). Then, as shown in FIG. 12(b), this substrate and the other substrate (a counter substrate 65 in this case) are laminated together and subjected to pressure and heat so as to adhere to each other. At this time, electrodes each in a protuberance form (protuberance electrodes 66) are formed on one or both of the foregoing substrates. As a result, only conductive particles 67 present in regions of the protuberance electrodes 66 are brought into contact with electrodes above and below the same, as shown in FIG. 12(c). This ensures electric connection between the electrodes on the upper and lower substrates. The regions where the protuberance electrodes 66 are absent (that is, regions corresponding to recessed portions) serve as places accepting bubbles caught upon the laminating of the substrates and portions of the anisotropic conductive adhesive 64 pushed out of between the electrodes when the gap between the substrates are narrowed to not greater than the diameter of the conductive particles 67.
In short, to ensure electric connection between the substrates with use of the anisotropic conductive adhesive 64 at a high yield, the protuberance electrodes 66 are preferably formed on at least one of the substrates.
Incidentally, in the case of the foregoing two-dimensional image detector, to form the protuberance electrode 66 on one of the substrates to be laminated together (the active matrix substrate 63, and the counter substrate 65 having the photoconductive film (semiconductor layer) 56), it is practical to form the protuberance electrodes 66 on the counter substrate 65 having the photocoductive film (semiconductor layer) 56. This is because, as described above, the active matrix substrate 63 is itself formed by repeatedly conducted microstructure processing steps, and to add the protuberance electrode forming step to such a substrate forming process is disadvantageous from the viewpoint of yield.
Therefore, to form the protuberance electrodes on the counter substrate 65 provided with the photoconductive film 56, a method of forming metal protuberances of Ni or the like with a height of 5 xcexcm to 20 xcexcm at desired positions on a surface of the photoconductive film (semiconductor layer) 56 by plating seems feasible.
In the case of the plating method, however, a problem of unevenness of deposited metal arises unless the concentration and temperature of the plating liquid is controlled. For example, according to experiments conducted by the applicant, in the case where Ni is deposited by electroless plating, the unevenness of deposited Ni is aggravated at a ratio of about 5%/1xc2x0 C.
In the case of the foregoing two-dimensional image detector, though depending on the purpose of application, a large substrate with a surface area of, for example, about 40 cmxc3x9750 cm is needed, and when the plating method is applied to such a large-scale substrate, it is difficult to strictly control the concentration and temperature of the plating liquid on the surface so as to achieve uniform distribution of concentration and temperature thereof. As a result, in the case where the protuberance electrodes are formed on the counter substrate by plating, thickness of the plating (that is, height of protuberance electrodes) on the surface varies, thereby leading to a drawback in that, when the active matrix substrate and the counter substrate are laminated together with use of the anisotropic conductive adhesive, regions in which vertical conductivity is achieved and regions in which vertical conductivity is not achieved are both obtained according to the variation in height of the protuberance electrodes.
The following description explains a problem common to the aforementioned first and second methods.
In the case where, to laminate together the substrates on which the active matrix substrate and the photoconductive film (semiconductor layer) have been formed respectively in advance, either (i) the anisotropic conductive adhesive of the first method is used as the connecting material or (ii) the conductive material is applied as the connecting material independently at each pixel according to the second method, connection electrodes and a charge blocking layer are sometimes provided on a surface of the photoconductive film (semiconductor layer). In this case, it is necessary to divide the connection electrodes and the charge blocking layer into independent units respectively corresponding to the pixels. Therefore, patterning of the connection electrodes and the charge blocking layer into such pixel units is needed like in the case of the pixel arrangement of the active matrix substrate, thereby resulting in an increase in the number of processing steps and a rise of the costs.
The first object of the present invention is to provide a method for manufacturing a two-dimensional image detector having a plurality of protuberance electrodes uniform in height which are formed on one and same substrate.
The second object of the present invention is to provide a method for manufacturing a two-dimensional image detector characterized in that pixels are formed on a connection surface of a semiconductor layer composing the two-dimensional image detector.
To achieve the above-described first object, a method for manufacturing a two-dimensional image detector of the present invention is a method for manufacturing a two-dimensional image detector which includes an active matrix substrate and a counter substrate, the active matrix substrate having a pixel-arrayed layer in which pixel electrodes and switching elements are arrayed in a lattice form substrate, the counter substrate having a semiconductor layer with photoconductivity and an electrode section formed so as to face substantially an entirety of a surface of the pixel-arrayed layer, the active matrix substrate and the counter substrate being electrically connected via a conductive material, and the method is characterized by including the step of (a) forming a plurality of protuberance electrodes on a connection surface of the counter substrate by blasting.
By the foregoing method, grooves can be formed in a predetermined pattern by blasting such as sandblasting, while the flatness of the substrate surface before the processing is maintained. Therefore, it is possible to easily form on one and same substrate a plurality of the protuberance electrodes uniform in height as compared with the protuberance electrodes obtained by the conventional plating. By laminating the counter substrate with a plurality of such protuberance electrodes uniform in height and the active matrix substrate together with use of the anisotropic conductive adhesive, a two-dimensional image detector hardly prone to connection defects can be realized, even in the case where electric connection has to be obtained densely in a large surface area.
Incidentally, another method of forming a plurality of the protuberance electrodes uniform in height on the basis of the identical principles is a method of forming the grooves on the substrate surface by etching. The etching, however, makes the manufacturing process more complicated since, in the case where a carrier blocking layer and connecting electrodes are present on the substrate surface, an etching liquid or an etching gas has to be changed so as to be suited to each film.
In formation of the grooves by sandblasting, on the other hand, since the grooves are physically formed by causing the ceramic particles to collide against the substrate surface, even in the case where the carrier blocking layer or the connecting electrodes, for example, are present on the substrate surface, the grooves can be formed by boring these films through one step. Therefore, this method is superior from the viewpoints of simplification of manufacturing process and lowering of costs.
The foregoing step (a) preferably includes the sub-steps of a. forming a mask having openings in a predetermined pattern on the electrode section composed of connecting electrodes, a carrier blocking layer, etc., b. processing the mask-covered surfaces of the connecting electrodes and semiconductor layer by blasting, so that at the openings of the mask the connecting electrodes and the carrier blocking layer are removed while grooves are formed on the semiconductor layer, and c. removing the mask.
With the foregoing arrangement, in addition to the effect of easier formation of a plurality of protuberance electrodes uniform in height on one and same substrate, the following effect can be achieved: the patterning of connecting electrodes formed on the semiconductor layer surface and the formation of the protuberances (grooves in a lattice form) can be carried out through one and same step, thereby making the manufacturing method superior from the viewpoints of simplification of manufacturing process and lowering of costs.
To achieve the aforementioned second object, another method for manufacturing a two-dimensional image detector of the present invention is a method for manufacturing a two-dimensional image detector which includes an active matrix substrate and a counter substrate, the active matrix substrate having a pixel-arrayed layer in which pixel electrodes and switching elements are arrayed in a lattice form substrate and the counter substrate having a semiconductor layer with photoconductivity and an electrode section formed so as to face substantially an entirety of a surface of the pixel-arrayed layer, the active matrix substrate and the counter substrate being electrically connected via a conductive material, and the method is characterized by including the step of (a) forming a plurality of pixels on a connection surface of the counter substrate by blasting.
The foregoing method, since pixels are formed by blasting, requires only a device which causes ceramic particles to collide against the substrate surface, and does not need an etching device. Moreover, in the case where a carrier blocking layer and a pixel electrode are present on the surface substrate, grooves can be easily formed by boring these films through one step. Therefore, this method is superior from the viewpoints of simplification of manufacturing process and lowering of costs.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.